EP3341153B1 - Ablative production device and method for a periodic line structure on a workpiece - Google Patents

Description

technical field

A technique for producing periodic structures using electromagnetic radiation is described. In particular, but not limited to, a device according to the preamble of claim 1 (see e.g CA2 281 039 A1 ), and a method for the production of periodic line structures by ablation.

State of the art

By structuring surfaces, surface areas can be marked visually or haptically. Furthermore, depending on the viewing angle, diffractive structures can appear differently colored or produce a color gradient. In addition to decorative purposes, structured surfaces can also be dirt-repellent, for example dewetting.

The document " Micron and Sub-Micron Gratings on Glass by UV Laser Ablation", J. Meinertz et al., Physics Procedia, Vol. 41, pages 708-712 , at the "Lasers in Manufacturing 2013" conference describes a conventional generation of parallel lines by means of a diffraction grating. However, a significant part of the radiant power of a processing laser is lost at the diffraction grating. In addition, a Schwarzschild optic with the main maximum of the diffraction grating hides another large part of the radiation power.

Furthermore, from the US2009/0046757 a laser irradiation device is known, by means of which positions of crystal grain boundaries produced during laser crystallization can be controlled.

demolition

Thus, one possible object of the present invention is to provide a technique for more efficiently fabricating microstructures.

According to one aspect, a device for the ablative production of a periodic line structure on a workpiece is provided. The device includes a pulsed laser for generating ablative light; one in the beam path of the ablative A light-arranged phase mask adapted to generate, by interference, a plurality of equidistant parallel lines in an object plane and to suppress a diffraction order parallel to the optical axis, the optical axis being perpendicular to the object plane; an imaging optics arranged on the optical axis with a cylindrical lens aligned parallel to the lines, which is designed to image the object plane into an image plane, the imaging optics reducing perpendicularly to the equidistant parallel lines; and a fixture configured to locate the workpiece in the image plane.

The equidistant parallel lines shown can create the periodic line structure on the workpiece by ablation. The periodic line structure on the workpiece can be periodic locally or in sections. Fabrication can result from multiple pulses of ablative light. For example, the periodic line structure can be produced in a contiguous area or in a plurality of contiguous areas on the workpiece.

The image plane can be perpendicular to the optical axis. The object plane and the image plane can be parallel.

A direction of the equidistant parallel lines defines a longitudinal direction. The optical axis and the longitudinal direction define a plane of symmetry. A direction perpendicular to the plane of symmetry defines a transverse direction.

Rays can be described (e.g. for a plane perpendicular to the optical axis) by an angle θ between the ray and the optical axis (or plane of symmetry). Alternatively or in addition, rays (e.g. for a plane perpendicular to the optical axis) can be described by a distance s (e.g. in the transverse direction) between the ray and the optical axis (or plane of symmetry).

The phase mask can modulate the phase of the laser light incident on it in the object plane. In the transverse direction, the phase mask can modulate a phase shift according to a rectangular profile onto the incident light. The phase mask can have a constant phase offset in the longitudinal direction (for each transverse direction).

The lines shown can be generated by interference. The imaged lines can be generated in the image plane by interference of two beams (a 2-beam interference). The phase mask can create the two beams. Each of the beams can correspond to a diffraction order. The (suppressed) diffraction order parallel to the optical axis is also referred to as "near-axis" light. Orders of diffraction that are not parallel to the optical axis are also referred to as "off-axis" light.

For example, the two interfering beams (e.g. the non-suppressed diffraction orders detected by the imaging optics) can comprise at least 75%, e.g. 80% to 90%, of the light (e.g. in terms of total radiant power after the phase mask or in terms of laser power). Alternatively or additionally, a portion of the near-axis light can be less than 10%, less than 5% or less than 1% (e.g. with regard to the total radiation power after the phase mask or with regard to the laser power). The phase mask can make it possible to provide as much light output as possible in the off-axis light (in the useful orders) for efficient use of the ablative light.

A high-contrast line structure can be made possible by the small proportion of the near-axis light. Optionally, a diaphragm (or a mirror) can be arranged between the phase mask and the imaging optics, e.g. directly on the imaging optics, to block (or reflect out of the beam path) the near-axis light. The two beams cannot overlap between the phase mask and the imaging optics in an area in front of the imaging optics. The aperture or the mirror can be arranged in the area.

An at least predominant portion of the off-axis light can increase the resolution of the image. For example, finer line structures can be produced. Alternatively or additionally, the suppression of the near-axis light can increase a depth of focus of the image. The depth of focus can be 50 µm to 250 µm, for example 100 µm. Alternatively or additionally, the suppression of the near-axis light and/or the predominance of the off-axis light can reduce or prevent the influence of a geometric (e.g. cylindrical or spherical) aberration of the cylinder lens on the imaging of the equidistant lines.

The object plane can essentially correspond to a plane of the phase mask and/or lie in a Talbot region of the phase mask.

The imaging optics can image the large number of equidistant parallel lines in the image plane. The imaging optics can be designed for reduced imaging. The mapping can be designed to reduce distances in the transverse direction. A distance between the imaged lines in the image plane can be smaller than the distance between the equidistant lines in the object plane.

The imaging optics can include further optical elements (e.g. lenses). For example, the imaging optics can comprise a doublet of two cylindrical lenses with a correspondingly combined refractive power.

The phase mask can be light efficient. The phase mask can have a transmittance of at least 75%, for example 80% to 90%. The light output of the phase mask can be considerably higher than the light output of a diffraction grating, for example several times. Furthermore, the use of the phase mask can suppress the near-axis light by interference without blocking or absorbing a main peak. The high transmittance can counteract heating or aging of the phase mask.

The laser and/or the phase mask can be designed to generate the plurality of equidistant lines in the object plane over a width X. The (e.g. the at least substantially non-suppressed) diffraction orders emanating from the phase mask can be spaced from the optical axis over the entire width X in the imaging optics. For example, the (e.g. the at least substantially non-suppressed) diffraction orders detected by the imaging optics can be spaced apart from the optical axis by more than the distance X.

The laser and/or the phase mask can be designed to generate the plurality of equidistant lines in the object plane over a first width. In the imaging optics, the orders of diffraction can be spaced apart from the optical axis by a second distance that corresponds to the first distance. The first width can extend transversely to the optical axis in the object plane.

The second width can extend transversely to the optical axis at the location of the imaging optics, for example in an imaging plane or main plane of the imaging optics. The second width may (eg, with a substantially collimated beam of ablative light) be substantially the same as the first width. The second Width may be increased (eg, in the case of a divergent beam of the ablative light) due to beam divergence compared to the first width. The correspondence of the second width with the first width can consist in a proportionality of the widths. The proportionality factor can be equal to or greater than 1.

The laser light can be designed for ablation. The light may be configured to cause localized ablation or alteration of material through exposure to high heat and/or plasma formation. The ablative light may be configured to ablate the workpiece held in the fixture in the image plane. A surface or a cutting plane of the workpiece can be arranged in the image plane by means of the holder.

The workpiece can include glass and/or be a glass body. The surface can be a glass surface. The glass can be a silicate glass.

The ablative effect of the light can be determined by its wavelength, pulse duration, pulse rate (or repetition rate), pulse energy, radiant power, fluence and/or intensity. The radiant power can affect the pulse power. The pulse power can be the ratio of pulse energy and pulse duration. The fluence can be the pulse energy per effective area. The intensity can be the pulse power per effective area.

The interfering beams can be the lowest non-suppressed diffraction orders. The spacing of the beams can be achieved by the beams not overlapping in the imaging optics.

The workpiece can be movably arranged in the image plane perpendicular to the lines. For example, structures can be produced over a large area. The large-area structures can be assembled by line-exact continuation of different ablation pulses. Alternatively or in combination, the large-area structures can be composed of ablation pulses that are essentially statistically distributed on the surface of the workpiece. In both cases, the ablation pulses can overlap on the surface of the workpiece.

The holder can be designed to move the workpiece parallel to the image plane, and for example perpendicular to the imaged equidistant lines, with a constant feed rate. For example, homogeneous large-scale structures are produced. The continuous feed rate can be essentially and/or at least temporarily constant.

A repetition rate r of the pulsed laser and the feed rate v of the stage can be synchronized. For example, the relationship v =r·b · n for an integer n can exist, at least temporarily, between the repetition rate and the feed rate v . Therein b can be a periodicity of the imaged equidistant lines.

The repetition rate can be 10 Hz to 5 kHz, for example 100 Hz to 1 kHz. The pulse duration can be 10 ns to 100 ns, for example 20 ns.

The line structure generated by consecutive pulses can overlap. An advance per pulse, b x n = v / r, can be smaller than an image-side width, Y. For example, the feed can be 0.9 x Y to 1.0 x Y.

Alternatively or additionally, the feed can be a fraction of the width Y on the image side. The feed b n can be Y / m for m =2, 3, 4,... A desired expansion of the line structure in the workpiece, for example parallel to the optical axis, can be achieved by multiple use of the ablative light.

The line structure can be continued up to a desired extent while maintaining the image-side periodicity b . Furthermore, the line structure can be continued to a desired surface by means of a meandering method.

Alternatively or additionally, a scanner can be arranged behind the imaging optics or as part of the imaging optics. The scanner can be designed to offset the imaged equidistant lines in one or two dimensions, for example for the aforementioned continuation of the periodic line structure.

The phase mask can be designed to suppress all even-numbered diffraction orders, including the zeroth order.

The non-suppressed diffraction orders can run symmetrically to the optical axis. Alternatively or additionally, the imaging optics can be arranged symmetrically to the optical axis. The imaging optics can be one of the phase mask (or the equidistant lines) and the plane defined by the optical axis must be mirror-symmetrical.

The imaging optics can detect two non-suppressed orders of diffraction, for example the first two orders of diffraction. The suppressed diffraction order can be the zeroth diffraction order. The phase mask may be arranged in a "+1/-1" configuration (e.g. with respect to the laser or its optical path). Furthermore, the phase mask can suppress a second diffraction order, e.g., any even diffraction order. The "+1/-1" configuration may allow for matching power or intensity of the two interfering beams of off-axis light.

Alternatively, the suppressed diffraction order can be a first diffraction order. The phase mask may be arranged (e.g. with respect to the laser or its optical path) in a "0/-1" configuration. The "0/-1" configuration can enable particularly efficient suppression of the near-axis light.

In all configurations, a third (or higher) diffraction order can be of negligible intensity and/or geometrically excluded (e.g. blocked) (e.g. by a lateral expansion of the imaging optics).

The imaging optics can only image in one dimension. The imaging optics can image perpendicularly to the equidistant lines.

The equidistant lines in the object plane and the imaged equidistant lines can have essentially the same length. The equidistant lines in the object plane and/or the imaged equidistant lines in the image plane can have a length of approximately 10 mm to approximately 50 mm, for example 20 mm.

The imaging optics can image the large number of equidistant lines in the image plane. The picture may shrink. The imaging optics reduce (in the image plane) perpendicularly to the equidistant lines. A distance between the imaged lines can be smaller than the distance between the equidistant lines in the object plane. A distance between the imaged lines (ie, their periodicity b ) can be smaller than the resolving power of the human eye.

A factor of reduction, Y / X=b / g, can be in the range 1/5 to 1/100, for example at 1/10 or at 1/80. A periodicity g of the equidistant lines in the object plane can be 2 μm to 200 μm, for example 25 μm. The periodicity of the equidistant lines in the object plane can be g = d or g = d /2 with a periodicity d of the phase mask. The set of imaged equidistant lines can have an image-side width Yin of the image plane of 10 μm to 1 mm, for example 100 μm.

The periodicity d of the phase mask can be 5 μm to 500 μm, for example 50 μm. The periodicity b of the imaged equidistant lines can be 0.5 μm to 25 μm, for example 2.5 μm or 5 μm.

10 to 1000, for example 50, 80 to 100, 200 or 500 equidistant lines can be imaged per pulse.

The periodic line structure produced according to the parallel lines shown can have a diffusively scattering and/or diffractively reflecting effect as a relief grating on the glass surface, for example for visible light.

Only a few outer zones of the cylindrical lens that are at a distance from the optical axis can be illuminated by the non-suppressed diffraction orders and/or contribute to imaging in the image plane.

Furthermore, the device can have an amplitude mask arranged between the imaging optics and the image plane. The amplitude mask can be spaced from the image plane. The amplitude mask can be moved together with or parallel to the workpiece.

According to a further aspect of the invention (see claim 10), a method for the ablative production of a periodic line structure on or in a workpiece is provided. The method includes the step of generating ablative light using a pulsed laser; the step of placing a phase mask in the optical path of the ablative light to produce a plurality of equidistant lines in an object plane and to suppress a diffraction order parallel to the optical axis by interference, the optical axis being perpendicular to the object plane; the step of mapping the object plane into an image plane using an on the optical Axis arranged and aligned parallel to the lines cylindrical lens; and the step of placing the workpiece in the image plane.

Brief description of the drawings

Fig. 1

zeigt ein schematisches Blockdiagramm einer Vorrichtung zur Herstellung einer periodischen Linienstruktur an einem Werkstück;

Fig. 2

zeigt schematisch eine Verteilung der Leistung des von einer Phasenmaske ausgehenden Lichts hinsichtlich Winkel und Abstand zur optischen Achse;

Fig. 3

zeigt schematisch ein Ausführungsbeispiel der Vorrichtung der Fig. 1;

Fig. 4

zeigt eine erste Konfiguration einer Phasenmaske für die Vorrichtung der Fign. 1 und 3 oder die Verteilung der Fig. 2; und

Fig. 5

zeigt eine zweite Konfiguration einer Phasenmaske für die Vorrichtung der Fign. 1 und 3 oder die Verteilung der Fig. 2.

Further features of the technology are described below using exemplary embodiments with reference to the accompanying drawings, in which:

1

shows a schematic block diagram of a device for producing a periodic line structure on a workpiece;

2

shows schematically a distribution of the power of the light emanating from a phase mask in terms of angle and distance to the optical axis;

3

shows schematically an embodiment of the device of FIG 1 ;

4

12 shows a first configuration of a phase mask for the device of FIG Figs. 1 and 3 or the distribution of 2 ; and

figure 5

FIG. 12 shows a second configuration of a phase mask for the device of FIG Figs. 1 and 3 or the distribution of 2 .

Detailed description

1 FIG. 1 shows a device, generally designated by reference numeral 100, for producing a periodic line structure. The device 100 comprises a pulsed laser 109 for generating ablative light 110, a phase mask 102 with an object plane 103 arranged in the beam path of the ablative light 110, imaging optics 104 arranged on an optical axis 101, and a holder 106 for a workpiece 108 in a Image plane 107 of the imaging optics 104 to be arranged.

Through interference, the phase mask 102 generates a first line structure in the object plane 103. The object plane 103 is perpendicular to the optical axis 101. The object plane is in the near field of the phase mask 102 on the side of the phase mask 102 facing away from the laser 109.

The object plane 103 may be spaced one Talbot length or one-half Talbot length (or an integer multiple of one-half or one-half Talbot length) from the plane of the phase mask.

In the far field of the phase mask 102 the interference suppresses a diffraction order parallel to the optical axis 101 . Two beams 116 and 118 (at least essentially) emanate from the phase mask 102 . Rays 116 and 118 are symmetrical to optical axis 101.

The imaging optics 104 images the object plane 103 in the image plane 107 by means of the positive refractive power of a cylindrical lens. The imaging optics 104 optionally include further optical elements, for example each with a positive refractive power. A doublet with two cylindrical lenses can, with the same combined refractive power, reduce aberrations compared to a single cylindrical lens with the same refractive power. The doublet can be asymmetrical. The cylindrical lenses can each be aligned with a convex side to the phase mask 102 .

The object plane 103, the imaging optics 104 and the image plane 107 are arranged relative to one another in such a way that the first line structure is imaged to form a reduced second line structure. The second line structure is created in the object plane 107 by interference of the two beams 116 and 118.

The ablative light can be monochromatic. The ablative light can include ultraviolet light. The pulsed laser 109 can generate ultraviolet light. The pulsed laser 109 may be an excimer laser such as an argon fluoride laser. A wavelength of the light may range from 126 nm to 351 nm. The wavelength of the light can be around 193 nm.

The first line structure and the second line structure each contain equidistant parallel lines, ie maxima of the intensity of the light. The lines are perpendicular to the drawing sheet 1 and define a longitudinal direction. The longitudinal direction and the optical axis 101 span a plane of symmetry. With respect to the plane of symmetry, an angle θ and a distance s are defined for each of beams 116 and 118, eg, in a plane perpendicular to the optical axis in imaging optics 104. Angle θ and/or distance s may have opposite signs on opposite sides of the plane of symmetry.

The imaging optics 104 can be designed to obtain a coherence and/or a relative phase position of the two beams 116 and 118 . An optical path length of the imaging optics 104 may depend (e.g. due to uncorrected optical elements, e.g. due to the cylindrical lens) on the angle θ of the rays 116 and 118 incident on the imaging optics.

The phase mask 102 can distribute the ablative light (or at least a majority of it) into discrete angles, e.g. into two defined angles corresponding to the two beams 116 and 118. The imaging optics 104 can (e.g. due to the illumination through the phase mask and/or due to a or several apertures) can only be used for the discrete angles. As a result, an influence of the angular dependence of the optical path length that is disadvantageous for sharpness and/or contrast can be avoided. A correction of the imaging optics, e.g. with regard to the angle dependency, can be omitted.

The two beams 116 and 118 can be symmetrical about the plane of symmetry. The two angles of rays 116 and 118 can be equal in magnitude. This allows the two beams 116 and 118 to travel the same optical path length between phase mask 102 and workpiece 106 for high-contrast interference.

2 shows schematically a power distribution 200 of the ablative light in a plane perpendicular to the optical axis 101 before or in the imaging optics 104. The distribution 200 of the power of the light is shown schematically with respect to the angle θ to the plane of symmetry and the distance s to the plane of symmetry. The distribution 200 shows a discretized distribution of the power with respect to the angle to the plane of symmetry.

One diffraction order defines a sharp angle θ>0 for the power 202 of the first ray 116. Another diffraction order defines a sharp angle θ<0 for the power 204 of the first ray 118. Entering the imaging optics 104 (at least relative to the power of the off-axis light 202 and 204) no near-axis light 206.

The cylindrical lens can have a circular cross section (for example in the plane of symmetry). The cylindrical lens is not necessarily corrected (eg with regard to geometric aberrations). Due to the discrete angular distribution 200, a correcting acylinder in the imaging optics 104 can be dispensed with. At least in exemplary embodiments, an optical path length of the imaging optics can depend (at least essentially) only on the angle of the rays incident on the imaging optics relative to the optical axis or to the plane of symmetry. The optical path length of the imaging optics can (at least essentially) be independent of the distance s from the optical axis 101 . This allows light from the phase mask to be imaged over a width X that overlaps with the optical axis 101 .

Alternatively or additionally, the width X can be small, for example the width X can be small in relation to the object distance G. Alternatively or additionally, the beams 116 and 118 can not overlap in the imaging optics 104, so that the width X is small in relation to the distance s to the plane of symmetry, as shown schematically for the power distribution 200. As a result, the requirement for the imaging optics 104 can be further reduced. For example, lighter or less expensive cylindrical lenses can be used in the imaging optics 104 .

3 shows a schematic sectional view of an exemplary embodiment of the device 100. The plane of FIG 3 The sectional image shown is parallel to the optical axis 101 and perpendicular to the equidistant lines 113.

in 3 In the exemplary embodiment of the device 100 shown, the phase mask 102 produces the first line structure 112 in the object plane 103. Irrespective of whether the first line structure 112 is present at all, the beams 116 and 118 are designed to produce the ablative line structure 113 by superimposition in the image plane 107.

the 3 is schematic for the sake of clarity. For example, the ray path can have a (in 3 not shown) have an intermediate focus, for example between an imaging plane 105 and the image plane 107. Alternatively, the intermediate focus in the case of convergent beams 116 and 118 can be between the object plane 103 and the imaging plane 105.

in 3 In the exemplary embodiment shown, the two beams 116 and 118 are (at least approximately) collimated beams, for example in that the beams 116 and 118 correspond to defined diffraction orders which are emitted by the phase mask at a defined diffraction angle θ to the optical axis will. The width X of beams 116 and 118 at reference number 114-2 may substantially correspond to the width X illuminated by laser 109 at reference number 114-1.

With slight divergence of beams 116 and 118, the width X at reference numeral 114-2 may be greater than the illuminated width X at reference numeral 114-1. For example, in the imaging plane 105 (or directly in front of the imaging optics 104), the width X can be at most 10% larger than the width X in the object plane 103 (or directly behind the phase mask 102).

The beams 116 and 118 enclose (at least approximately) a defined angle +θ or −θ with the optical axis 101 on the input side of the imaging optics 104 . The imaging of the beams 116 and 118 by means of the imaging optics 104 uses only an (at least approximately) discrete angular range. In addition, when the paths of the beams 116 and 118 are symmetrical with respect to the optical axis 101 and the imaging optics 104 are arranged symmetrically with respect to the optical axis 101, the imaged angles are (at least approximately) of the same absolute value.

Imaging with only a single angular magnitude may improve interference of the imaged rays and/or reduce imaging optics 104 requirements. For example, a particularly sharp and/or high-contrast line structure 113 can be made possible, although a distortion of a wave front of the beams 116 and 118 would be expected with a simple cylindrical lens.

The line structure 113 can have a rectangular profile in the image plane 107 transversely to the equidistant lines. A high sharpness of the image can correspond to steep flanks of the rectangular profile. Alternatively, the line structure 113 can have a sinusoidal intensity distribution in the image plane 107 transverse to the equidistant lines.

In the case of high contrast, intensity minima of the line structure 113 can be essentially radiation-free, so that the workpiece 108 remains unprocessed in the troughs of the intensity distribution.

3 shows an embodiment with separate beams 116 and 118 in the imaging optics. A distance W of the beams 116 and 118 from the plane of symmetry can be large in relation to the width X of the beams 116 and 118.

the Figs. 4 and 5 show a section 300 of the device 100 for exemplary embodiments of the phase mask 102. The imaging optics 104 comprise a cylindrical lens aligned parallel to the lines 112. A radius of the cylindrical lens is large in relation to the distance W of the rays 116 and 118 from the plane of symmetry in the imaging plane 105. The cylindrical lens 104 is not necessarily acylindrically corrected.

4 12 shows a first configuration of the phase mask 102 in which the beams 116 and 118 correspond to the two first diffraction orders +1 and -1. The first configuration may allow the power of the ablative light 110 to be distributed symmetrically between the two beams 116 and 118 .

figure 5 12 shows a second configuration of phase mask 102 in which ray 116 corresponds to the zeroth diffraction order and ray 118 corresponds to a first diffraction order. The second configuration can allow full rejection of near-axis light without lossy glare.

The cylindrical lens 104 allows for extensive processing of the workpiece 108 in the longitudinal direction. A machining range is further expanded by shifting the workpiece by means of the holder 106 . The holder 106 continuously shifts the workpiece 108 in the image plane 107 with uninterrupted pulse operation of the laser 109. Alternatively or additionally, the holder rotates the workpiece, for example in order to machine a curved surface. The rotation is about the instantaneous normal intersection point of the current edit region.

For example, the workpiece is continuously shifted in the transverse direction, so that with each pulse of the laser 109 in an overlapping area of the processing with an area processed by the previous pulse, the parallel lines shown are congruent. Since the product of pulse duration and displacement speed is small compared to the width of the imaged lines, time-consuming acceleration and deceleration processes can be avoided.

By using the phase mask 104, a fluence of the laser 109 can be used almost completely for processing the workpiece 108.

Workpiece 108 may include glass. The technology can be used for labeling or surface treatment of hobs with glass ceramics, eyeglass lenses or primary packaging.

In each of the exemplary embodiments, an amplitude mask can be arranged between the workpiece 108 and the imaging optics 104, e.g. in the image plane 107. The amplitude mask can enable a display composed of the line structure. The representation can include a graphic, lettering, a pictogram or a machine-readable code. The machine-readable code can be structured one-dimensionally (e.g. as a barcode) or structured two-dimensionally (e.g. as a QR code).

Claims (10)

1. Apparatus (100) for ablatively producing a periodic line structure on a workpiece (108), comprising:

   a pulsed laser (109) configured to generate ablative light (110);

   a phase mask (102) disposed in the beam path of the ablative light (110) and configured to generate by interference a plurality of equidistant parallel lines (112) in an object plane (103) and to suppress a diffraction order parallel to the optical axis (101), the optical axis (101) being perpendicular to the object plane (103);

   characterised by the following features:

   an imaging optic (104) disposed on the optical axis (101) and having a cylindrical lens aligned parallel to the lines (112) and configured to image the object plane (103) into an image plane (107), the imaging optic (104) reducing perpendicular to the equidistant parallel lines (112); and

   a support (106) configured to position the workpiece (108) in the image plane (107).
2. Apparatus according to claim 1, wherein the unsuppressed diffraction orders (116, 118) include discrete angles with the optical axis (101) and the cylindrical lens has no acylindrical correction.
3. Apparatus according to claim 1 or 2, wherein the unsuppressed diffraction orders (116, 118) and the imaging optic (104) are symmetrical about the optical axis (101).
4. Apparatus according to any one of claims 1 to 3, wherein the phase mask is configured to generate the plurality of equidistant parallel lines (112) in the object plane (103) over a first width (114-1), and wherein the imaging optic (104) is arranged on the optical axis (101) such that unsuppressed diffraction orders (116, 118) in the imaging optic (104) are spaced from the optical axis (101) over a second width (114-2) corresponding to the first width (114-1).
5. Apparatus according to any one of claims 1 to 4, wherein the workpiece is movably arranged in the image plane (107) perpendicular to the lines (112, 113).
6. Apparatus according to any one of claims 1 to 5, wherein the support (106) is configured to displace the workpiece (108) parallel to the image plane (107) and perpendicular to the imaged equidistant lines (113) at a steady feed rate.
7. Apparatus according to claim 6, wherein a repetition rate, r, of the pulsed laser (109) and the feed rate, v, of the support (106) are synchronised such that $$v=r\cdot b\cdot n\mathrm{for}\mathrm{an}\mathrm{integer}n,$$

   

   where b is a periodicity of the imaged equidistant lines (113).
8. Apparatus according to claim 6 or 7, wherein a feed between two successive pulses is an integer fraction of an image side width (115).
9. Apparatus according to any one of claims 1 to 8, further comprising:
   an amplitude mask arranged between the imaging optic (104) and the image plane (107).
10. Method of ablatively producing a periodic line structure on a workpiece (108), comprising:

    generating ablative light (110) by means of a pulsed laser (109);

    disposing a phase mask (102) in the beam path of the ablative light (110) for generating a plurality of equidistant parallel lines (112) in an object plane (103) and for suppressing a diffraction order parallel to the optical axis (101) by interference, wherein the optical axis (101) is perpendicular to the object plane (103);

    imaging the object plane (103) into an image plane (107) by means of a cylindrical lens arranged on the optical axis (101) and aligned parallel to the lines (112); and

    position the workpiece (108) in the image plane (107).

Images